Image pickup apparatus

ABSTRACT

An apparatus includes pixels each having a transistor that transfers a charge of a photoelectric conversion unit, an amplification unit that receives the transferred charge, a scanning unit that supplies, to the transistor, a conductive pulse, a non-conductive pulse, and an intermediate-level pulse having a peak value between the conductive pulse and the non-conductive pulse, a generating unit that generates an image signal using a signal based on a charge transferred in response to the conductive and intermediate-level pulses, and a control unit that changes at least one of a pulse width of the intermediate-level pulse and the peak value in accordance with information on the detected temperature. The conductive and intermediate-level pulses are supplied to the transistor during a light shielding period of the photoelectric conversion unit.

CROSS-REFERENCED OF RELATED APPLICATIONS

This application claims the benefit U.S. application Ser. No. 12/574,618filed Oct. 6, 2009 and of Japanese Patent Application No. 2008-262998filed Oct. 9, 2008, which are hereby incorporated by reference herein intheir entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image pickup apparatus and, morespecifically, to the technology of extending a dynamic range.

2. Description of the Related Art

There are technologies of extending a dynamic range in image pickupapparatuses. Japanese Patent Laid-Open No. 2006-197382 (hereinafterreferred to as Patent Document 1) discloses an image pickup apparatusthat aims at extending a dynamic range without a decrease in imagequality. Specifically, the image pickup apparatus includes atransfer-gate control portion that controls a potential of a transfergate to introduce a part of charges flowing from a photoelectricconversion portion into a floating diffusion portion and an image-signalgenerating portion that generates an image signal based on chargesstored in the photoelectric conversion portion and charges flowed intothe floating diffusion portion.

Japanese Patent Laid-Open No. 2008-099158 (hereinafter referred to asPatent Document 2) discloses a configuration in which intermediatevoltages having the same voltage value are supplied a plurality of timesas a control voltage from a driver circuit to a gate electrode of atransfer transistor in synchronization with column selection and at thattime signal charges transferred by the transfer transistor are read outat least twice. Each of the intermediate voltages supplied to thetransfer transistor is transferred to a floating diffusion (FD) region,and a potential in the FD region is read out as a signal level. In adifferent embodiment, a configuration using a mechanical shutter isdisclosed. In that configuration, a dummy transfer is performed duringthe opening of the mechanical shutter, and an intermediate voltage istransferred and read out during the closing of the mechanical shutter.

One possible method for extending a dynamic range in an image pickupapparatus is an increase in saturation charge quantity of thephotoelectric conversion portion. However, if the saturation chargequantity of the photoelectric conversion portion is simply increased,not all charges may be read out in a readout circuit disposed downstreamof the photoelectric conversion portion or a signal based on a readoutcharge may be unable to be used in image formation.

In Patent Document 1, light is incident on the photoelectric conversionportion, charges flowed from the photoelectric conversion portion duringthe period of generating signal charges for use in image formation (theexposure period) in the photoelectric conversion portion are transferredto the FD region. That is, on the assumption of generation of chargesexceeding the saturation charge quantity of the photoelectric conversionportion during the exposure period, charges are transferred during theexposure period. Thus, if strong light is given after a last readoutoperation during the exposure period and charges are stored near thesaturation charge quantity of the photoelectric conversion portion, notall charges may be used in image formation depending on the dynamicrange of the downstream readout circuit.

In Patent Document 2, if strong light is entered, a dummy transfer isperformed and a part of generated charges is ejected through a resetswitch. Therefore, there may be an issue in terms of signal continuity.In addition, no discussion is made about variations in transferefficiency from the photoelectric conversion portion depending ontemperature conditions of the exterior and the gain of the readoutcircuit disposed downstream of the photoelectric conversion portion.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, an apparatus includes aplurality of pixels, an amplification unit, a scanning unit, agenerating unit, a detecting unit, and a control unit. Each of theplurality of pixels has a photoelectric conversion unit and a transfertransistor that transfers a charge of the photoelectric conversion unit.The amplification unit is configured to receive the transferred charge.The scanning unit is configured to supply, to a gate of the transfertransistor, a conductive pulse, a non-conductive pulse, and anintermediate-level pulse having a peak value between the conductivepulse and the non-conductive pulse. The generating unit is configured togenerate an image signal using a signal based on a charge transferred inresponse to the conductive pulse and the intermediate-level pulse. Thedetecting unit is configured to detect a temperature. The control unitis configured to change at least one of a pulse width of theintermediate-level pulse and the peak value in accordance withinformation on the detected temperature. The conductive pulse and theintermediate-level pulse are supplied to the transfer transistor duringa light shielding period of the photoelectric conversion unit.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings, in which like reference characters designate the sameor similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a solid-state image pickup apparatus.

FIG. 2 is an equivalent circuit diagram of pixels of the solid-stateimage pickup apparatus.

FIG. 3 is a block diagram of an image pickup apparatus according to afirst embodiment of the present invention.

FIG. 4 illustrates driving pulses according to the first embodiment.

FIGS. 5A to 5C illustrate potentials according to the first embodiment.

FIG. 6 is a block diagram of an image pickup apparatus according to asecond embodiment of the present invention.

FIGS. 7A to 7H illustrate relationships between surface illumination andoutput according to the second embodiment.

FIGS. 8A to 8E illustrate distributions of pixel signals according to athird embodiment of the present invention.

FIG. 9 illustrates a relationship between a pixel signal and noiseaccording to the third embodiment.

FIGS. 10A to 10D illustrate how signal processing is performed accordingto a fourth embodiment of the present invention.

FIG. 11 is an equivalent circuit diagram of a solid-state image pickupapparatus according to a fifth embodiment of the present invention.

FIG. 12 illustrates driving pulses according to the fifth embodiment.

FIGS. 13A to 13E illustrate potentials according to the fifthembodiment.

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention are described below with referenceto the accompanying drawings. First, a block diagram of a solid-stateimage pickup apparatus that can be shared by the embodiments describedbelow and an equivalent circuit diagram of pixels are described withreference to FIGS. 1 and 2.

Referring to FIG. 1, a pixel region 101 has a plurality of pixelsarranged in a matrix form. A vertical scanning unit 102 is a circuit foruse in scanning pixels in the pixel region row by row or rows by rows.The vertical scanning unit 102 can be constructed using a shift registeror a decoder.

A column circuit 103 can also function as a readout circuit. The columncircuit 103 performs predetermined processing on a signal read out fromthe pixel region 101 by a scan executed by the vertical scanning unit102. The column circuit 103 can include, for example, a CDS circuit thatsuppresses noise of a pixel, an amplification unit that amplifies asignal from a pixel, and an analog-to-digital (A/D) converter thatconverts analog signals from a pixel into digital signals.

A horizontal scanning circuit 104 sequentially scans pixels column bycolumn or columns by columns to read out a signal subjected topredetermined processing by the column circuit 103. The horizontalscanning circuit 104 can be constructed using a shift register or adecoder, similar to the vertical scanning unit 102.

A signal processing unit 105 performs predetermined processing on asignal output from the solid-state image pickup apparatus.

Although being omitted in FIG. 1, wiring for transmitting an opticalsignal or a driving signal exists between elements.

FIG. 2 is an equivalent circuit diagram of pixels arranged in the pixelregion. For the sake of clarity, the number of pixels contained in thepixel region 101 is 9 consisting of 3 rows by 3 columns. However, thenumber of pixels is not limited to 9, so more than 9 pixels may bearranged.

A photodiode 201 functions as a photoelectric conversion unit andphotoelectrically converts incoming light to generate signal charges. Atransfer transistor 202 functions as a transfer unit and transferscharges generated by the photoelectric conversion unit to anamplification unit, which will be described below. A floating diffusion(FD) region 203 functions as a part of an input section of theamplification unit, and charges generated by the photoelectricconversion unit are transferred to the FD region 203. Other elementsconstituting the input section of the amplification unit can be a gateof an amplification transistor, which will be described below, and aconductor that electrically connects the FD region and the gate of theamplification transistor.

An amplification transistor 204 functions as an amplification unit. Theamplification transistor 204 and a constant-current supply (not shown)constitute a source follower circuit. The gate of the amplificationtransistor is electrically connected to the FD region. A resettransistor 205 functions as a reset unit. The reset transistor includesa source connected to the FD region and the gate of the amplificationtransistor and a drain to which a reset voltage is supplied. A selectiontransistor 206 functions as a selection unit. The selection transistorcontrols operations of the amplification unit to select a pixel. Theselection unit can also be used as the resent unit and can controlselection and non-selection of a pixel using a voltage supplied to thegate of the amplification transistor.

A transfer driving line PTX is used in supplying a pulse for controllingconduction and non-conduction of the transfer transistor. A resetdriving line PRES is used in supplying a pulse for controllingconduction and non-conduction of the reset transistor. A selectiondriving line PSEL is used in supplying a pulse for controllingconduction and non-conduction of the selection transistor. Drivingpulses are supplied to these driving lines from the vertical scanningunit. That is, the vertical scanning unit can function as a scanningunit for the transfer transistor and can supply a pulse to the gate ofthe transfer transistor.

In FIG. 2, the reset unit, the amplification unit, and the selectionunit are disposed in each pixel. However, they can be shared by aplurality of photoelectric conversion units. A charge storing unit maybe disposed between each of the photoelectric conversion units and theFD region. In this case, the configuration includes a first transferunit that transfers a charge from the photoelectric conversion unit tothe charge storing unit and a second transfer unit that transfers acharge from the charge storing unit to the FD region. In this case, anintermediate-level pulse, which will be described below, may be suppliedto either the first transfer unit or the second transfer unit.

Specific configurations and driving methods are described below.

FIG. 3 is a block diagram of an image pickup apparatus according to afirst embodiment of the present invention.

A temperature detecting unit 302 measures a temperature in or near asolid-state image pickup apparatus 301. For example, a diode can be usedin the temperature detecting unit 302. The temperature detecting unit302 disposed within the solid-state image pickup apparatus may also bearranged in an outside position at which the temperature near thesolid-state image pickup apparatus can be measured. A control unit 304controls the solid-state image pickup apparatus in response toinformation on the temperature from the temperature detecting unit 302and a control signal from a central processing unit (CPU) 303. Avariable-voltage supply source 305 supplies a voltage corresponding tothe temperature to the solid-state image pickup apparatus in response toa control signal from the control unit 304. Supplying this voltage tothe vertical scanning unit illustrated in FIG. 1 can vary a peak valueof an intermediate-level pulse in accordance with the temperature.Alternatively, the pulse width of an intermediate-level pulse may bevaried.

An operation flow according to the present embodiment is describedbelow. First, signal charges are accumulated in the photoelectricconversion unit. After the completion of a predetermined exposureperiod, the accumulation is completed. After that, the temperature ofthe solid-state image pickup apparatus itself or near the solid-stateimage pickup apparatus is acquired by the temperature detecting unit. Alookup table that records voltage information corresponding to theacquired temperature information is accessed, and control is performedsuch that a voltage corresponding to the temperature is supplied fromthe variable-voltage supply source. After that, a readout operation isexecuted, as described below.

The energy that signal charges have varies depending on the temperature.If a potential barrier in a charge path between the photoelectricconversion unit and the FD region does not vary even when thetemperature varies, the number of charges transferred to the FD regionis different. In contrast, in the present embodiment, the temperaturedetecting unit is provided, and, in response to a signal from thetemperature detecting unit, at least one of a peak value of a pulsesupplied to the transfer transistor and a pulse width thereof is variedor switched. Specifically, if the temperature rises, the number ofcharges is increased even with the same pulse peak value. Accordingly,in order to achieve a constant quantity of transferred charges,irrespective of temperature, it is useful to make the pulse peak valuesmaller or the pulse width narrower for a high temperature and make thepulse peak value larger or the pulse width wider for a low temperature.

Driving pulses in the present embodiment are illustrated in FIG. 4.Potential statuses in elements are illustrated in FIG. 5. In FIG. 4, PTXindicates a pulse supplied to the gate of the transfer transistor; PRESindicates a pulse supplied to the gate of the reset transistor; PSELindicates a pulse supplied to the gate of the selection transistor. Thefigures in parentheses indicate pixel row numbers. The solid-state imagepickup apparatus according to the present embodiment has a mechanicalshutter. When the mechanical shutter is in an open state, light entersthe photoelectric conversion unit; when the mechanical shutter is in aclosed state, no light enters the photoelectric conversion unit. Themechanical shutter can specify an exposure period. In FIG. 4, for themechanical shutter, the hatched section indicates the closed state, andthe blank section indicates the open state.

PTS indicates a sampling pulse used in capturing a signal into anoptical signal storing unit contained in the column circuit; PTNindicates a sampling pulse used in capturing a signal into anoise-signal storing unit contained in the column circuit. Noise signalscan be pixel offsets of the reset transistor and the amplificationtransistor, random noise, or, if the column circuit includes theamplification unit, offsets of this column amplification unit.

In response to a high-level pulse, the transistors become conductive orperform sampling. Therefore, a high-level pulse can also be called aconductive pulse. In contrast, a low-level pulse can also be called anon-conductive pulse.

First, at T1, the reset transistor in a conductive state supplies ahigh-level pulse to PTX, and charges in the photoelectric conversionunit are reset. At this time, the mechanical shutter is in a closedstate.

At T2, the mechanical shutter is brought into an open state, thuscausing light to enter the photoelectric conversion unit. At this time,a pulse supplied to each of the transfer transistors is a low-levelpulse that makes the transfer transistor non-conductive.

At T3, the mechanical shutter is brought into a closed state. This endsa period for which the photoelectric conversion unit generates signalcharges.

At T4, a low-level pulse is supplied to PRES in the first pixel row, anda high-level pulse is supplied to PSEL. Here, the pulses are suppliedsimultaneously. However, they can be supplied at different times. Tosuppress kTC noise in the resent unit, at least in a period for which ahigh-level pulse is supplied to PTN that samples a noise signal, alow-level pulse is supplied to PRES.

At T5, a high-level pulse is supplied to PTN, and a noise signal in thefirst pixel row is stored in the column circuit.

At T6 a, an intermediate-level pulse between a high-level pulse and alow-level pulse is supplied to PTX in the first pixel row. Only chargesthat exceed a potential barrier produced by the supply of theintermediate-level pulse in the photoelectric conversion unit aretransferred to the FD region (first step). As is clear from the drawing,the first step is performed during the light shielding period, where themechanical shutter shields the photoelectric conversion unit from light.

At T6 b, a low-level pulse is supplied to PTX in the first pixel row.

At T7, a high-level pulse is supplied to PTS, and a signal based on acharge transferred in response to the supply of the intermediate-levelpulse to the transfer transistor in the first pixel row is stored in thecolumn circuit.

At T8, a low-level pulse is supplied to PSEL in the first pixel row, anda high-level pulse is supplied to PRES in the first pixel row. Thisoperation resets charges transferred to the FD region in the first step.

At T9, a high-level pulse is supplied to PTN, and a noise signal in thefirst pixel row is stored in the column circuit.

At T10 a, a high-level pulse is supplied to PTX in the first pixel row(second step). It is useful that a pulse having a peak value sufficientfor fully transferring charges in the photoelectric conversion unit tothe FD region be supplied. As is clear from the drawing, the second stepis performed during the light shielding period, where the mechanicalshutter shields the photoelectric conversion unit from light.

At T10 b, a low-level pulse is supplied to PTX in the first pixel row.

At T11, a high-level pulse is supplied to PTS, and a signal based oncharges transferred in the second step in the first pixel row is storedin the column circuit.

At T12, a low-level pulse is supplied to PSEL in the first pixel row,and a high-level pulse is supplied to PRES.

A signal obtained by the supply of a high-level pulse to the transfertransistor (second step) and a signal obtained by the supply of anintermediate-level pulse to the transfer transistor (first step) areadded in the signal processing unit 105, which functions as an imagesignal generating unit. This enables most of charges caused by thephotoelectric conversion unit to be handled as signals for use in imageformation, irrespective of the dynamic range of the readout circuitdisposed downstream of the photoelectric conversion unit.

The operations from T4 to T12 are repeated for each pixel row, thusreading out signals for one frame.

In the present embodiment, a transfer using an intermediate-level pulseat T6 a to T6 b (first step) is performed only once. However, it can beperformed a plurality of times.

Next, a potential status at a time illustrated in FIG. 4 is describedwith reference to FIGS. 5A to 5C. In the illustrated potential statuses,the hatched sections indicate a state in which the mechanical shuttershields the photoelectric conversion unit from light.

FIG. 5A illustrates a potential status when a low-level pulse causing anon-conductive state is supplied to the transfer transistor. In thisstatus, the mechanical shutter is in an open state, an exposure periodhas been completed, and signal charges are stored in the photoelectricconversion unit. That potential status exists in the periods T3 to T6 a,T6 b to T10 a, and T10 b to T12 in the first pixel row in FIG. 4. In theother pixel rows, the potential status illustrated in FIG. 5A exists inthe period T3 to T12. In that status, a potential barrier sufficient forblocking signal charges stored in the photoelectric conversion unit fromflowing into the FD region is present in a charge path between thephotoelectric conversion unit and the FD region.

FIG. 5B illustrates a status in which an intermediate-level pulse issupplied to the transfer transistor. That status corresponds to theperiod T6 a to T6 b in FIG. 4. In this period, the potential barrier inthe charge path between the photoelectric conversion unit and the FDregion is also in an intermediate potential status between a conductivestate and a non-conductive state. In that potential status, only signalcharges that exceed the intermediate potential barrier of the signalcharges stored in the photoelectric conversion unit are transferred tothe FD region. The saturation level of the amplification unit indicatesthe maximum number of charges when the source follower of a pixel or thedownstream readout circuit is not saturated.

FIG. 5C illustrates a status in which a high-level pulse is supplied tothe transfer transistor. In this status, the potential barrier betweenthe photoelectric conversion unit and the FD region is low sufficientlyto allow signal charges in the photoelectric conversion unit to betransferred to the FD region. It is useful that a pulse having a peakvalue that allows signal charges in the photoelectric conversion unit tobe fully transferred to the FD region be supplied. This can easily bedetermined from the relationship between the potential in thephotoelectric conversion unit and the potential in the FD region duringthe transfer.

Here, a peak value of an intermediate-level pulse is described.Generally, it is useful that all the signal charges produced in thephotoelectric conversion unit be used in image formation. However, ifthe capacity of the photoelectric conversion unit is large and thesaturation charge quantity is also large, even when many signal chargesare produced in the photoelectric conversion unit, the dynamic range maybe limited by the saturation of the downstream readout circuit. Examplesof the readout circuit having such limitations include a source followercircuit of a pixel, a column amplification unit disposed in the columncircuit, and an A/D conversion unit. Here, the dynamic range of thesource follower circuit being the pixel amplification unit is describedas one example. When the amplification unit forms the source followercircuit using the amplification transistor and the constant-currentsupply, if many signal charges are transferred from the photoelectricconversion unit to the FD region, a decrease in the potential in the FDregion is also large. Because the FD region is electrically connected tothe gate of the amplification transistor, if the decrease in thepotential in the FD region makes the difference between the gatepotential and the source potential of the amplification transistor lowerthan a threshold voltage (Vth), a source follower operation is inactive.If so, a signal cannot be read out. Even when charges fewer than thedynamic range of the source follower circuit are read out, if a highgain is provided in the column amplification unit, the saturation of thereadout circuit is determined by the limitation on the input dynamicrange of the column amplification unit.

Accordingly, the peak value of the intermediate-level pulse can be setat a value at which charges not exceeding the dynamic range of thereadout circuit can be transferred. When predetermined processing isperformed in each readout circuit every time a readout is executed, evenif the saturation charge quantity of the photoelectric conversion unitis so large that it exceeds the dynamic range of the readout circuit,all signal charges produced in the photoelectric conversion unit can beread out. Because such an operation is performed after an exposureperiod is completed in the photoelectric conversion unit, i.e., during alight shielding period of the photoelectric conversion unit, chargesaccumulated in the photoelectric conversion unit do not increase, so allsignal charges accumulated in the photoelectric conversion unit can betransferred. Additionally, because the peak value of anintermediate-level pulse is varied with the temperature, signal chargesin the photoelectric conversion unit can be read out while adverseeffects of temperature changes are suppressed.

The present embodiment is particularly effective when the combination ofthe dynamic range of the photoelectric conversion unit and that of thecharge storing unit is larger than the dynamic range of the readoutcircuit disposed downstream of the photoelectric conversion unit.

With the present embodiment, even when the temperature changes in thesolid-state image pickup apparatus or in the vicinity thereof, a readoutis executable without limitations imposed on the dynamic range of thereadout circuit disposed downstream of the photoelectric conversionunit.

A second embodiment is an example in which an intermediate-level pulsedescribed in the first embodiment is varied in accordance with the gainof the readout circuit disposed downstream of the photoelectricconversion unit. Here, examples of the readout circuit disposeddownstream of the photoelectric conversion unit include an amplificationunit disposed in a pixel, a column amplification unit disposed in thecolumn circuit, and an output amplification unit used to output a seriessignal into which a parallel signal from the column circuit is convertedto the outside.

FIG. 6 is a block diagram of an image pickup apparatus including asolid-state image pickup apparatus according to the present embodiment.

Reference numeral 601 represents a solid-state image pickup apparatus.Reference numeral 602 represents a control circuit. Reference numeral603 represents a CPU. Reference numeral 604 represents avariable-voltage supply source. The gain of an amplification circuitcontained in the readout circuit disposed downstream of thephotoelectric conversion unit is set by the control circuit 602 and theCPU 603. The gain can be controlled in response to, for example,sensitivity switching or a readout-speed change.

An operation flow according to the present embodiment is describedbelow. First, signal charges are accumulated in the photoelectricconversion unit. After a predetermined exposure period is completed, theaccumulation is completed. After that, a lookup table that recordsvoltage information corresponding to the gain of the amplificationcircuit contained in the readout circuit disposed downstream of thephotoelectric conversion unit is accessed, and control is performed suchthat a voltage corresponding to the gain is supplied from thevariable-voltage supply source. After that, the above-described readoutoperation is executed. Additionally, a temperature detecting unit may beprovided for correction with the temperature, as described in the firstembodiment. That is, the peak value or the pulse width of anintermediate-level pulse may be determined from both the gain of theamplification circuit and the ambient temperature.

Changes in the peak value or the supply time of an intermediate-levelpulse when the gain of the readout circuit is variable are describedbelow with reference to FIGS. 7A to 7H.

FIG. 7A illustrates a dependence of an output voltage on the incidentlight quantity (photoelectric conversion characteristic) when a firstintermediate-level pulse is supplied. FIG. 7B illustrates aphotoelectric conversion characteristic when a second intermediate-levelpulse is supplied. FIG. 7C illustrates a photoelectric conversioncharacteristic when a high-level pulse is supplied. FIG. 7D illustratesan output signal characteristic after addition after charges aretransferred to the FD region in response to each readout pulse. FIGS. 7Eto 7G illustrate photoelectric conversion characteristics and FIG. 7Hillustrates an output signal characteristic when the peak value of apulse is varied in accordance with the gain of the readout circuit.Specifically, FIGS. 7E to 7H illustrate an example in which, when thegain of the readout circuit is changed from a first gain to a secondgain higher than the first gain, the peak value of a pulse is reducedfrom that illustrated in FIGS. 7A to 7C.

When the gain of the readout circuit is variable, the incident lightquantity up to saturation of the circuit (saturation light quantity)differs from gain to gain of the readout circuit. The light quantities904 a, 904 b, and 904 c represent ones occurring when the gain of thereadout circuit is low. The light quantities 906 a, 906 b, and 906 crepresent ones occurring when the gain of the readout circuit is high.When the gain of the readout circuit is low, the saturation lightquantity is high, as indicated at 905 a, 905 b, and 905 c. Accordingly,all of the signal charges transferred to the FD region can be used assignals for use in image formation. However, when the gain of thereadout circuit is high, the saturation light quantity is low, asindicated at 907 a, 907 b, and 907 c. Accordingly, a part of the signalcharges transferred to the FD region exceeds the input dynamic range ofthe readout circuit, and this means that a part of information isabsent. As a result, as illustrated in FIG. 7D, a photoelectricconversion characteristic obtained after signals in regions A, B, and Cillustrated in FIGS. 7A to 7C are combined is stepped, as indicated at909. That is, the obtained photoelectric conversion characteristicundesirably has a deadband. For reference, when there is no absence, alinear characteristic is obtainable, as indicated at 908.

In such a case, as illustrated in FIGS. 7E to 7G, the peak value of anintermediate-level pulse can be limited to a level lower than thatillustrated in FIGS. 7A to 7C. Alternatively, the pulse supply time canbe shortened to restrict transient movement of charges and reduce thenumber of charges transferred when an intermediate-level pulse is used.Therefore, even when pulses are supplied and read out three times,similar to the case illustrated in FIGS. 7A to 7C, the photoelectricconversion characteristics illustrated in FIGS. 7E to 7G are obtainable.The photoelectric conversion characteristic obtained after these signalsare combined is illustrated in FIG. 7H. In FIG. 7H, there is no absencein information, unlike FIG. 7D, so no undesired deadband is presentafter the signals in the regions A, B, and C are combined.

Because the number of charges that can be read out at a time is limited,the saturation light quantity after combination is reduced. Thereduction in saturation light quantity can be suppressed by an increasein the number of times in response to an intermediate-level pulse andthe peak value of a supplied intermediate-level pulse as needed.

With the present embodiment, even when the gain of the readout circuitdisposed downstream of the photoelectric conversion unit changes,signals having photoelectric conversion characteristics being continuouseven after combination are obtainable without the occurrence ofdeadbands.

A feature of a third embodiment is that the gain of the amplificationcircuit contained in the readout circuit is switched for each signalread out in response to each of a high-level pulse causing the transfertransistor conductive and an intermediate-level pulse (may be aplurality of pulses). An increase in the gain of the readout circuitraises the signal-to-noise ratio (S/N), and as a result, enableshigh-sensitive image capture of a subject in low-light conditions. Ifthe gain of the amplification circuit is set high when a signal is readout in response to an intermediate-level pulse, the number of signalcharges that can be read out at a time is reduced. To read out chargesin the photoelectric conversion unit as many as possible, anintermediate-level pulse is supplied more times. In addition, when theamplification circuit has a high gain, the readout speed is oftenreduced.

To address this, the gain with respect to a signal read out in responseto the supply of a high-level pulse that cause the transfer transistorconductive is set higher than the gain with respect to a signal read outin response to the supply of an intermediate-level pulse.

FIGS. 8A to 8E illustrate relationships between pixel signals of pixelsand noise according to the present embodiment. FIG. 8A illustrates adistribution of charges occurring in the photoelectric conversion unitof each of the pixels in response to incident light. In FIG. 8A, thesaturation in the photoelectric conversion unit and the boundary betweencharges transferred in response to an intermediate-level pulse andcharges transferred in response to a high-level pulse are indicated bythe dotted lines. The saturation charge number in the photoelectricconversion unit can be 100,000e⁻, for example. The quantity of chargestransferred in response to an intermediate-level pulse can be 90,000e⁻,for example. The quantity of charges transferred in response to ahigh-level pulse can be 10,000e⁻, for example.

FIG. 8B illustrates a portion A extracted from FIG. 8A. The extractedportion relates to signal charges transferred in response to anintermediate-level pulse. The random noise in the step of transferringcharges in response to an intermediate-level pulse can be represented asV_(RN1)(mVrms).

FIG. 8C illustrates a portion B extracted from FIG. 8A. The extractedportion relates to signal charges transferred in response to ahigh-level pulse. Here, the gain at the downstream part is G. In thiscase, the random noise can be represented as V_(RN2)(mVrms).

FIG. 8D illustrates the portion B extracted from FIG. 8A. The extractedportion relates to signal charges transferred in response to ahigh-level pulse. Here, the gain at the downstream readout circuit is1/G. That is, the gain is lower than that in FIG. 8C. In this case, therandom noise can be represented as V_(RN2)/G(mVrms).

FIG. 8E illustrates a combined condition. The random noise can be((V_(RN1))²+(V_(RN2)/G)²)^(0.5), and a readout with low noise ispossible.

FIG. 9 illustrates a relationship between an incident light quantity andan output (i.e., sensor output) from the solid-state image pickupapparatus.

As described above, in addition to the features obtained in otherembodiments, the improving S/N of the sensor without sacrificing thereadout speed is obtainable.

A fourth embodiment differs from other embodiments in that it ispossible to switch between addition and non-addition of a signalobtained in the state where a high-level pulse causing the transfertransistor conductive is supplied to a signal obtained in response to anintermediate-level pulse.

The non-execution of an addition of the above-described signal obtainedin response to the supply of a high-level pulse if a threshold is notexceeded can prevent addition of random noise. The details are describedbelow with reference to FIGS. 10A to 10D. FIG. 10A illustrates a signalobtained when signal charges are transferred to the FD region inresponse to a first intermediate-level pulse. FIG. 10B illustrates asignal obtained when signal charges are transferred to the FD region inresponse to a second intermediate-level pulse. FIG. 10C illustrates asignal obtained when signal charges are transferred to the FD region inresponse to a conductive pulse. FIG. 10D illustrates an image obtainedafter signals are combined.

In FIGS. 10A and 10B, most of the signals indicated by the squares inthe hatched region have a random noise component. If addition isperformed, S/N is degraded. In contrast, in FIG. 10C, as indicated inthe shaded region, the signals are at a level where they are to be usedas signals for use in image formation. For example, for a subject inlow-light conditions, signal charges are accumulated (e.g., in anaccumulation unit) in only the photoelectric conversion unit withoutexceeding the potential barrier produced by an intermediate-level pulse.Accordingly, if a transfer operation using an intermediate-level pulseis performed in such a state, the noise is simply increased.

To address this, if a signal based on charges transferred in response toan intermediate-level pulse does not exceed a predetermined threshold,the signal obtained when the transfer is performed using theintermediate-level pulse may not be used as signals for use in imageformation. Alternatively, if a signal obtained when the transfer isperformed using a conductive pulse is not smaller than a predeterminedthreshold, it may be determined that there is a signal from the chargestoring unit, and the signal may be used as a signal for use in imageformation. Additionally, a unit configured to monitor the incident lightquantity for each pixel may be provided, and a case where the signal isused in image formation and a case where the signal is not used in imageformation may be switched depending on the incident light quantity tothe pixel.

FIG. 11 is an equivalent circuit diagram of pixels arranged in the pixelregion according to a fifth embodiment. For the sake of clarity, thenumber of pixels contained in the pixel region 101 is 9 consisting of 3rows by 3 columns. However, the number of pixels is not limited to 9. Ina pixel 21, a photodiode (PD) 2 functions as the photoelectricconversion unit. The PD 2 has an anode connected to a fixed potential(for example, the ground) and a cathode connected to a first terminal ofa charge storing unit 3 through a first transfer transistor 8functioning as a first transfer unit. The charge storing unit 3 has asecond terminal connected to a fixed potential (for example, theground). The first terminal of the charge storing unit 3 is connected toan FD region 4 through a second transfer transistor 9 functioning as asecond transfer unit. The FD region 4 is connected to a gate terminal ofan amplification transistor 12 functioning as a part of theamplification unit. The gate of the amplification transistor 12functions as an input section of the amplification unit. The gate of theamplification transistor 12 is connected to a pixel power line through areset transistor 10 functioning as the reset unit. Each of the transfertransistors can be a metal oxide semiconductor (MOS) transistor.

A selection transistor 11 functioning as the selection unit has a drainterminal, which is a first main electrode, connected to the pixel powerline and a source terminal, which is a second main electrode, connectedto the drain, which is a first electrode, of the amplificationtransistor 12. When an active signal SEL is input, both the mainelectrodes of the selection transistor become conductive. This cause theamplification transistor 12 to form the source follower circuit togetherwith a constant-current supply (not shown) disposed in a vertical signalline OUT and causes a signal corresponding to a potential of the gateterminal being a control electrode of the amplification transistor 12 toappear in the vertical signal line OUT. A signal based on the signalappearing in the vertical signal line OUT is output from the solid-stateimage pickup apparatus, and it forms an image signal through the signalprocessing circuit.

In FIG. 11, the reset unit, the amplification unit, and the selectionunit are provided in each pixel. However, they can be shared by aplurality of pixels. Alternatively, a configuration that does not havethe selection unit and that can select a pixel using a potential of theinput section of the amplification unit can be used.

The present invention is applicable to a configuration that includes thecharge storing unit between the photoelectric conversion unit and the FDregion, one example of such a configuration being described above.

It is particularly useful that the structure of the charge path betweenthe photoelectric conversion unit and the charge storing unit use aconfiguration having the following features, i.e., a configuration inwhich charges can be transferred from the photoelectric conversion unitto the charge storing unit in a state where a low-level pulse causingthe first transfer unit non-conductive.

A specific example configuration is the one in which, when the firsttransfer unit is a MOS transistor, the MOS transistor has a buriedchannel structure. In that configuration, the potential barrier has alower portion in a location lower than the surface even in anon-conductive state, the lower portion existing in only that location.In this case, the charge transfer unit may also be in a state where aconstant voltage is supplied without the performance of active controlduring the accumulation of signal charges. That is, a fixed potentialbarrier may also be provided without the function as the transfer unit.Immediately before the completion of the accumulation, control isperformed such that the height of the potential barrier is reduced, andsignal charges remaining in the photoelectric conversion unit aretransferred to the charge storing unit.

With this configuration, it is possible to transfer substantially all ofthe signal charges produced by photoelectric conversion when light isincident on the photoelectric conversion unit to the charge storing unitwithout being accumulated in the photoelectric conversion unit.Accordingly, the photoelectric conversion units contained in all thepixels can have substantially the same time of accumulation of charges.When the MOS transistor is not conductive, holes are accumulated in thechannel surface and a channel to which a charge is transferred ispresent in a portion lower than the surface by a predetermined depth.Accordingly, adverse effects of a dark current on the insulating filminterface can be reduced.

From another point of view, in a period for which signal charges areaccumulated in the photoelectric conversion unit and the charge storingunit, it can be said that the potential barrier in the charge pathbetween the photoelectric conversion unit and the charge storing unit islower than the potential barrier in the charge path between thephotoelectric conversion unit and another region. The potential usedhere indicates the potential with respect to a signal charge. Forexample, when an OFD region is provided, that potential barrier can havea potential lower than the potential between the photoelectricconversion unit and the OFD region.

It is particularly useful that, in a period where signal charges areaccumulated in the charge storing unit made of a charge-coupled device,charges having an opposite polarity to the signal charges be accumulatedin the surface of the charge storing unit by application of a potentialto the counter electrode through the insulating film to suppressgeneration of a dark current on the semiconductor surface on which thecharge storing unit is disposed.

With this configuration, a dark current caused by the charge storingunit can be further reduced. At the same time, because it is notnecessary to inject opposite conductivity type impurities for addressinga dark current into the surface of the charge storing unit, a portionthat deals with charge storing can be formed in a shallower areaadjacent to the surface than the photoelectric conversion unit (e.g., aphotodiode). Accordingly, the ability to store charges per unit volumecan be enhanced such that the ability to store charges can be severaltimes as high as that of a traditional configuration in which aphotodiode functions to store charges.

In terms of driving, in one signal charge generation period, a signalcharge transferred from the photoelectric conversion unit to the chargestoring unit is stored in the charge storing unit and is used as animage signal. That is, after the beginning of one signal chargegeneration period, a signal is read out to the outside of a pixelwithout a reset operation in the charge storing unit. One signal chargegeneration period is determined so as to be common to the photoelectricconversion units when one frame image is captured.

A specific configuration and driving method according to the presentembodiment of the present invention are described below. A pixelconfiguration in this case is based on a configuration in which a pixelhas a structure in which the first transfer unit is a buried channel MOStransistor and the charge storing unit is made of a charge coupleddevice. The description is provided using, as an example, aconfiguration in which a transistor is used as each of the first andsecond transfer units.

FIG. 12 illustrates a driving pulse according to the present embodiment.FIG. 13 illustrates a potential. The present embodiment is distinct inthat how a signal charge generation period for one frame is determined.In contrast to the first embodiment, in which the mechanical shutter isused, the signal charge generation period is specified by the use of anelectronic shutter in the present embodiment.

The driving pulse illustrated in FIG. 12 is described. Mainly, thedescription is provided while focusing on points different from thefirst embodiment.

First, at T1, a high-level pulse is supplied to each of the resettransistor and the first to third transistors, thus causing thetransistors conductive. The charges in the photoelectric conversionunit, the charge storing unit, and the FD region are reset.

At T2, a low-level pulse is supplied to the third transfer transistor,and a signal charge generation period is started. When the mechanicalshutter is not used, although light is incident on the photoelectricconversion unit, the OFD region and the photoelectric conversion unitare made non-conductive by the third transfer transistor and a statewhere the photoelectric conversion unit and the charge storing unitstore charges is determined as the signal charge generation period. Atthis time, the pulse supplied to each of the first and second transfertransistors is a low-level pulse causing each of the transfertransistors non-conductive.

At T3 a, a high-level pulse is supplied to the third transfertransistor, and the signal charge generation period is completed. Thatis, the state is the one in which signal charges produced in thephotoelectric conversion unit are ejected to the OFD region. At the sametime, an intermediate-level pulse between a high-level pulse and alow-level pulse is supplied to the second transfer transistor (firststep). Only a part of the signal charges in the photoelectric conversionunit and the charge storing unit that exceeds a potential barrier inresponse to the supply of the intermediate-level pulse is supplied tothe FD region.

At T3 b, a low-level pulse causing a non-conductive state is supplied tothe second transfer transistor.

At T4 a, a high-level pulse causing a conductive state is supplied tothe second transfer transistor (second step). It is useful that a pulsehaving a peak value at which signal charges in the charge storing unitcan be fully transferred to the FD region be supplied. At this time, thefirst transfer transistor is in a state where a low-level pulse issupplied.

At T4 b, a low-level pulse causing a non-conductive state is supplied tothe second transfer transistor.

At T5 a, a high-level pulse causing a conductive state is supplied toeach of the first and second transfer transistors (third step).

At T5 b, a low-level pulse causing a non-conductive state is supplied toeach of the first and second transfer transistors.

The third transfer transistor is in a conductive state, and all of thesignal charges in the photoelectric conversion unit have been ejected tothe OFD region. Accordingly, the driving at T5 a and T5 b may not beperformed.

After that, the signals obtained in the first to third steps are addedin the signal processing unit 105. Most of the signal charges producedby photoelectric conversion in the photoelectric conversion unit can behandled as signals for use in image formation, irrespective of thedynamic range of the readout circuit disposed downstream of thephotoelectric conversion unit and the charge storing unit.

Repeating the operations from T3 a to T5 b for each pixel row enablessignals for one frame to be read out.

The transfer in response to an intermediate-level pulse (first step) atT3 a to T3 b is performed only once. However, the transfer may beperformed a plurality of times.

In the present embodiment, the period for which the third transfertransistor is in a conductive state, that is, the period for whichcharges of the photoelectric conversion unit are ejected to the OFDregion can be regarded as a light-shielding period. During this period,a high-level pulse and an intermediate-level pulse are supplied to thetransfer transistor.

Next, a potential status at each time illustrated in FIG. 12 isdescribed with reference to FIGS. 13A to 13E.

FIGS. 13A and 13B illustrate potential statuses at T2 to T3 aillustrated in FIG. 12.

FIG. 13A illustrates a status in which a few quantity of light isincident. In this status, charges are stored in the photoelectricconversion unit without exceeding the potential barrier between thephotoelectric conversion unit and the charge storing unit determined bythe peak value of a pulse supplied to the first transfer transistor.Here, a low-level pulse is supplied to the first transfer transistor.The potential barrier is controlled so as to have a relatively low levelsuch that signal charges produced in the photoelectric conversion unitare immediately moved to the charge storing unit. Such a status can beachieved by the use of a buried channel MOS transistor as the firsttransfer transistor, as described above.

FIG. 13B illustrates a status in which a part of signal charges producedin the photoelectric conversion unit exceeds the potential barrierdetermined by the first transfer transistor, the exceeding part ofsignal charges is moved to the charge storing unit, and the moved signalcharges are stored also in the charge storing unit. The dotted lineindicates the height of the potential barrier in a state where alow-level pulse is supplied to the first transfer transistor.

FIG. 13C illustrates a potential status at T3 a to T3 b illustrated inFIG. 12. In this potential status, a low-level pulse is supplied to thefirst transfer transistor and an intermediate-level pulse is supplied tothe second transfer transistor (first step). Due to this operation,signal charges above the dotted line illustrated in FIG. 13B and a partof the signal charges accumulated in the charge storing unit aretransferred to the FD region. Because the third transfer transistor isin a conductive state, the potential barrier between the photoelectricconversion unit and the OFD region is low and a signal charge producedin the photoelectric conversion unit after the transfer is transferredto the OFD region. That is, although a charge is produced inphotoelectric conversion, it is ejected to the OFD region, so the periodcan be regarded as a period for which no signal charge is generated.

FIG. 13D illustrates a potential status at T4 a to T4 b illustrated inFIG. 12. In this state, a low-level pulse is supplied to the firsttransfer transistor and a high-level pulse is supplied to the secondtransfer transistor (second step). Due to this operation, a signalcharge remaining after the signal charges stored in the charge storingunit are transferred in the second step is transferred to the FD region.

FIG. 13E illustrates a potential status at T5 a to T5 b illustrated inFIG. 12. In this state, a high-level pulse is supplied to each of thefirst to third transfer transistors (third step). As described above,the third step may be provided; it is not necessarily required.

With the present embodiment, although charges transferred to the FDregion are slightly reduced, the dynamic range is extended and theconfiguration can be simple without the provision of a mechanicalshutter. This is also useful for high-speed operation.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications and equivalent structures and functions.

1. An apparatus comprising: a plurality of pixels each having aphotoelectric conversion unit; a floating diffusion region; a chargestoring unit; a first transfer transistor configured to transfer chargesgenerated at the photoelectric conversion unit to the charge storingunit; a second transfer transistor configured to transfer charges storedat the charge storing unit to the floating diffusion region; and a resettransistor electrically connected to the floating diffusion region, theapparatus further comprising: a scanning unit configured to supply, to agate of the second transfer transistor, a conductive pulse, anon-conductive pulse, and an intermediate-level pulse having a peakvalue between the conductive pulse and the non-conductive pulse, whereinthe intermediate-level pulse is supplied when the reset transistor is ina non-conductive state.
 2. The apparatus according to claim 1, furthercomprising a generating unit configured to generate an image signalusing a signal based on a charge transferred in response to theconductive pulse and the intermediate-level pulse.
 3. The apparatusaccording to claim 1, further comprising a detecting unit configured todetect a temperature.
 4. The apparatus according to claim 3, furthercomprising a control unit configured to change at least one of a pulsewidth and the peak value of the intermediate-level pulse in accordancewith information on the detected temperature.
 5. The apparatus accordingto claim 1, wherein the conductive pulse and the intermediate-levelpulse are supplied to the second transfer transistor during a lightshielding period of the photoelectric conversion unit.
 6. The apparatusaccording to claim 1, wherein image signals for one frame are generatedusing signals based on charges transferred in response to theintermediate-level pulses supplied to the second transfer transistor aplurality of times.
 7. The apparatus according to claim 4, furthercomprising an amplification unit, a gate of the amplification unit beingconnected to the floating diffusion region; and a readout circuitdisposed downstream of the amplification unit, the readout circuitincluding an amplifier circuit having a variable gain, wherein at leastone of the peak value and the pulse width of the intermediate-levelpulse is determined based on information on the temperature detected bythe detecting unit and the gain of the amplifier circuit.
 8. Theapparatus according to claim 1, further comprising an amplificationunit, a gate of the amplification unit being connected to the floatingdiffusion region; and a readout circuit disposed downstream of theamplification unit, the readout circuit including an amplifier circuithaving a variable gain, wherein the gain of the amplifier circuit withrespect to a signal read out in response to the conductive pulse ishigher than the gain of the amplifier circuit with respect to a signalread out in response to the intermediate-level pulse.
 9. The apparatusaccording to claim 1, wherein using and not-using a signal read out inresponse to the intermediate-level pulse in image formation areswitchable depending on a quantity of light incident on the pixels.